Apparatus and method for super-cooled operation of a cryostat with low quantities of coolant

ABSTRACT

A cryostat arrangement ( 1 ′) having a vacuum container ( 2 ) and an object ( 4 ) to be cooled, which is arranged inside the vacuum container. A neck tube ( 8 ) leads to the object, and a cooling arm ( 10 ) of a cold head ( 11 ), around which a closed cavity ( 9 ) is formed, is arranged in the neck tube, which is sealed off fluid-tight in relation to the object and is filled with cryogenic fluid in normal operation. A thermal coupling element ( 15 ) couples the cryogenic fluid in the cavity to the object. A pump device ( 14 ), to which the cavity is connected via a valve ( 13 ) and with which the cavity is pumped out if the cold head fails. A monitoring unit ( 17 ) monitors the cooling function of the cold head, and activates the pump device to pump out the cavity if the cooling function of the cold head drops.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a Continuation of PCT/EP2017/071340, filed on Aug. 24, 2017, which claims foreign priority under 35 U.S.C. § 119(a)-(d) to German Application No. 10 2016 218 000.2 filed on Sep. 20, 2016. The entire contents of both the PCT application and the priority application are hereby incorporated into the present Continuation by reference.

FIELD OF THE INVENTION

The invention relates to a cryostat arrangement having a vacuum container and an object to be cooled, in particular a superconducting magnetic coil system or a cryogen container, wherein the object to be cooled is arranged inside the vacuum container, wherein the vacuum container has a neck tube which leads to the object to be cooled, wherein a cooling arm of a cold head is at least partially arranged in the neck tube, wherein a closed cavity, which is sealed fluid-tight in relation to the object to be cooled, is formed around the cooling arm, wherein the cavity is at least partially filled with a cryogenic fluid in normal operation, and wherein a thermal coupling element is provided, suitable for thermal coupling of the cryogenic fluid in the cavity with the object to be cooled.

BACKGROUND

Such a cryostat arrangement is known from DE 10 2014 218 773 A1 (=reference [1]).

Nuclear magnetic resonance (=NMR) apparatuses, in particular NMR spectrometers and NMR tomographs, require strong magnetic fields, which are often generated with superconducting magnet coils. The superconducting magnet coils have to be operated at a cryogenic temperature. The magnet coils are typically arranged in a cryostat for this purpose. The cryostat has an evacuated container (“vacuum container”), in which an object to be cooled is arranged, often also enclosed by a radiation shield. The object to be cooled can be the magnet coil itself (“cryo-free” system), or also a cryogen container, in which a cryogenic liquid (such as liquid helium) and the magnet coil are arranged.

In cryogenic technology, refrigeration machines are usually used for cooling objects—for example, superconducting magnet coils—which dissipate the heat from the apparatus containing an object to be cooled through a cold head. The object to be cooled is generally cooled using an active cooling system, typically comprising a pulse tube refrigerator or a Gifford-McMahon refrigerator. Active cooling systems reduce the consumption of costly liquid helium, increase the availability of the NMR apparatus, and can also contribute to reducing the overall height. The active cooling system can be designed as single-stage or also multistage. In multistage systems, a warmer cooling stage usually cools the thermal radiation shield and a colder cooling stage cools the object to be cooled. The known refrigeration machines typically operate using helium gas as a coolant, which is compressed in a compressor and relaxed in the cold head of the cryostat, for example, as in the pulse tube refrigerator. Cold head and compressor are generally connected to one another by two pressure lines. The cold head is connected to the components to be cooled either directly mechanically or by a contact medium (for example, cryogenic gas or cryogenic liquid) or by both, to ensure a good heat transfer.

However, if the compressor fails entirely or partially—for example, due to a technical defect or power failure—the previously cooled components heat up. The cold head of the cryostat then represents a substantial heat bridge between the components to be cooled and the outside world in this situation. In the case of a superconducting magnet, the superconducting current can flow practically without resistance over extremely long periods of time in its persistent operating mode. Heating of the magnets, in contrast, results, after a certain time, in the so-called “quench” of the persistent operating mode: The magnet reaches the critical transition temperature predefined by the superconductor material at a certain time, becomes normal-conducting, and loses its high magnetic field at the same time—generally suddenly.

If such a failure of the active cooling system takes place, the superconducting magnet coil system is to be able to remain below the transition temperature at least until a repair of the active cooling system can be performed and/or the power/water failure has been overcome. A loss of the superconducting state makes renewed cooling of the superconducting magnet coil system necessary, which is linked to substantial expenditure.

In the most typical configurations of cryostats having active cooling, as described, for example, in US 2007/089432 A, the cooling arm of a cold head protrudes into a neck tube of a vacuum container. The neck tube is open toward a cryogen container, in which a superconducting magnet coil is arranged in liquid helium. Helium recondenses at the lowermost cooling stage of the cooling arm and drips back into the cryogen container. Similar cryostats are known from US 2010/298148 A, US 2007/022761A, DE 10 2004 012 416 B4, or US 2007/051115 A.

To make the available time between a failure of an active cooling system and a service intervention as long as possible, the thermal masses (i.e., the mass multiplied by the specific heat capacity) in the cryostat, for example, of a radiation shield or a cryogen container including cryogenic liquid, can be selected to be large, which increases the overall height and the overall weight of the cryostat, however. Externally acquired liquid helium can also be refilled in the case of cryostats having cryogen containers, in order to replace evaporated helium; however, this is very expensive.

Conducting a part of the evaporating gas from the cryogen container along the refrigerator when the refrigerator is not operating, for example, during transportation, and thus reducing the heat load of the cooling arm, is known from U.S. Pat. No. 8,950,194 B2.

U.S. Pat. No. 8,729,894 B2 (=reference [2]) describes a magnet system for an MRI system, which comprises a vacuum pump connected to the cryogen tank, with which the cryogen can be pumped out to reduce the pressure in the cryogen tank, so that the cryogen cools down during the charging process, in order to arrive at a temperature level of approximately 2.5 K-3 K. In this magnet system, the entire cryostat is filled with cryogen. The controller for the pump can be, for example, a pressure sensor or a temperature sensor according to U.S. Pat. No. 8,729,894 B2.

Finally, DE 10 2014 218 773 A1, which was cited at the outset, discloses an MCV (minimal condensed volume) system for operating a cryostat. The object to be cooled (the magnet coil) is stored dry here, i.e., without helium. The cryogenic liquid, generally liquid helium, is located in a cavity in the neck tube between the cold head and a contact surface in relation to the object, wherein an equilibrium of gaseous and liquid helium forms as a function of pressure and temperature. In the normal case, the cryostat is operated at normal pressure. However, the operating pressure can also be at 200 mbar, i.e., in the partial vacuum range in relation to the atmospheric pressure.

The operation using liquid helium at partial vacuum has the advantage that the operating temperature is below 4.2 K, which results in a significantly better utilization of the superconductor. In the case of NbTi conductors, the Ic/B curve shifts, for example, by 2 to 3 Tesla toward higher fields if the conductor is cooled from 4.2 to 2 K.

A safety unit, with which a cooling measure is automatically initiated upon failure of the cold head, is not described in DE 10 2014 218 773 A1.

All discussed cryostat arrangements from the prior art have the disadvantage, inter alia, of the relatively high design temperature, generally around 4.2 K, with which a superconducting magnet coil to be cooled has to be designed for its operation. Accordingly, higher costs, a higher weight, and larger dimensions and usually a lower reliability in the event of power failure or coolant water failure thus result.

SUMMARY

One object of the present invention is to reduce the heat load from the cold head on the object to be cooled in the case of a failure of the refrigerating machine in an operationally-reliable and significant manner. In the case of a cryostat arrangement of the type described at the outset—in particular for subcooled operation of the object to be cooled below 4.2 K—the invention proposes a noncomplex technical solution, fully automatically without the necessity of the intervention of an operator. Already installed units can be retrofitted in a particularly simple manner. Furthermore, the object to be cooled can be kept automatically in the vicinity of the operating temperature for the longest possible bridging period of time.

This object is achieved by the present invention in that the cryostat arrangement comprises a pump device, to which the cavity is connected via a valve and with which the cavity can be pumped out in the event of a failure of the cooling function of the cold head, and a monitoring unit is provided, which monitors the cooling function of the cold head and which is designed to independently activate the pump device in the event of a drop of the cooling function of the cold head such that the cavity is pumped out.

Therefore—in particular at an operating temperature of approximately 3 K—in a CF (=cryogen free) system, during the charging procedure and in the event of failure of the cooling system, the required cooling of the object to be cooled can be maintained at least during a very substantially extended bridging period of time. Significantly reducing the design temperature of a magnet coil to be cooled and thus saving substantial costs is therefore enabled. A safety system for cryostats is provided by the present invention, with which it is possible, by maintaining a low pressure in a region of the cryostat in which a liquid cryogen is provided, to facilitate the transition into the gas phase and thus use the heat of vaporization in order to keep the cryostat subcooled. This system is preferably used in the MCV (=minimal condensed volume) variant, wherein the cryogen is pumped out in a cavity between the object to be cooled and the neck tube region.

Due to the reduction of the heat introduction linked thereto, the time until the magnet coil reaches its critical temperature upon failure of the active refrigerator and becomes normal-conducting is substantially extended. This time span is an essential specification of superconducting magnets.

EMBODIMENTS AND REFINEMENTS OF THE INVENTION

A class of embodiments of the cryostat arrangement according to the invention is in some cases preferred, which is distinguished in that a pressure sensor is connected to the cavity, in particular is arranged in the cavity, the output signal of which is fed into the monitoring unit, and the monitoring unit activates the pump device to pump out the cavity as soon as the output signal of the pressure sensor exceeds a predefined first threshold value P_(max), wherein in particular 100 mbar≤P_(max)≤500 mbar. In this way, cryogenic fluid is prevented from being pumped out even in the event of very short failures (<15 min), since it takes a certain amount of time until the pressure rises from the operating pressure to the threshold value.

The pressure is directly correlated with the desired temperature in the cavity. If the pressure is reduced to the specified range, the temperature in the cavity decreases accordingly. If one proceeds, for example, from the P/T diagram of helium as a cryogenic fluid, it is thus recognizable that a pressure of 240 mbar corresponds to a temperature of approximately 3 K and at a pressure of 100 mbar, the resulting temperature would be approximately 2.5 K.

Furthermore, a pressure sensor is cost-effective and can be reliably used in continuous operation.

In advantageous refinements of this class of embodiments, the pump device pumps out the cavity after its activation because of exceeding the first threshold value P_(max) only until the output signal of the pressure sensor falls below a predefined second threshold value P_(min), wherein in particular 75 mbar≤P_(min)≤300 mbar.

For this two-point regulation, P_(min)<P_(max) applies. Using this type of shutoff device, it is ensured that the cryogenic fluid is not pumped out further and therefore the duration of the cooling procedure is not shortened more than necessary by pumping out the cavity in the event of failure of the cooling function. When the cooling function begins again, a shutoff device which deactivates the pump device is also necessary.

In a further advantageous embodiment of the cryostat arrangement according to the invention, the activatable valve is embodied as a regulating valve.

The pressure in the cavity may thus be exactly regulated. The pumping out procedure can be optimized in such a way that the desired target temperature is accurately maintained. On the one hand, pumping out the available helium reserve excessively rapidly is thus avoided (this would reduce the holding time), on the other hand, it is also ensured that the coil does not heat up beyond the maximum allowable temperature.

In particularly simple refinements of this embodiment, the pump device is designed to be operated at constant speed and/or pumping capacity.

Such pumps are widespread and are particularly inexpensive and reliable.

In a further class of preferred embodiments of the invention, the pump device is designed in such a way that it can be operated at variable, preferably settable speed and/or pumping capacity. Such a solution would be selected in combination with an on-off valve (or possibly also entirely without an automatic valve). Regulating the speed of the pump is particularly favorable energetically, since throttle losses do not occur in a half-open regulating valve. Moreover, regulating the flow rate by varying the pump speed is more accurate than by using a regulating valve.

These embodiments can be equipped with an ON/OFF valve as an activatable valve in refinements which are again designed rather simply.

Such valves are widespread and are particularly cost-effective and reliable.

To make the safety system provided by the invention as autonomous as possible, so that it can start up even in the event of failure of the power supply, in particularly preferred embodiments of the cryostat arrangement according to the invention, the pump device comprises an electrically operated suction pump buffered by an autonomous power source, preferably with a battery.

Alternatively, instead of a mechanical pump, a cryogenically cooled sorption pump can also be used, which does not require any power in operation and functions completely passively. Such a cryopump is preferably integrated into the cryostat arrangement in order to achieve the most compact construction possible.

In the case of the use of a cryopump as a pump device, it does not have to be activated separately, since it is permanently active. Opening the valve is sufficient to activate the cryopump.

The pumping cold surfaces of the cryopump are preferably thermally coupled to the object to be cooled, in particular wherein a connecting line from the cavity to the cold surfaces to be pumped extends completely inside the vacuum container.

A cryopump has no moving parts, because of which it is particularly reliable. Moreover, it functions without the supply of electrical energy, i.e., a battery can be omitted.

To make the cryostat arrangement according to the invention as autonomous as possible, it is expedient if, after the failure of the cooling function, the possibility exists of refilling the loss of cryogenic fluid. The cryostat arrangement therefore preferably comprises a supply line, which is connected to the cavity, so that when the cooling function is put back into operation, the loss of cryogenic fluid is refillable.

It is advantageous if the supply line is equipped with a pressure reducer. On the one hand, the filling procedure can be carried out in a controlled manner, without an overpressure arising, because the cryogenic fluid is typically introduced in the gaseous state, wherein it can condense on the cold head. If the cryogenic fluid is supplied from the exterior, it possibly has a significantly higher temperature than desired in the cryostat, a controlled supply is therefore advantageous.

A method for operating a cryostat arrangement is also in the scope of the present invention, in particular an above-described cryostat arrangement according to the invention, having a vacuum container and an object to be cooled, in particular a superconducting magnetic coil system or a cryogen container, wherein the object to be cooled is arranged inside the vacuum container, wherein the vacuum container has a neck tube which leads to the object to be cooled, wherein a cooling arm of a cold head is at least partially arranged in the neck tube, wherein a closed cavity, which is sealed fluid-tight in relation to the object to be cooled, is formed around the cooling arm, wherein the cavity is at least partially filled with a cryogenic fluid in normal operation, and wherein a thermal coupling element is provided, suitable for thermal coupling of the cryogenic fluid in the cavity with the object to be cooled. The method according to the invention is characterized in that in the event of failure of the cooling function of the cold head, the cavity is pumped out via a pump device in such a way that the pressure in the cavity does not exceed a predefined first threshold value P_(max).

A variant of the method according to the invention is very particularly preferred, in which the cavity is pumped out via a pump device so that the pressure in the cavity does not fall below a predefined second threshold value P_(min)<P_(max).

Refinements of the method according to the invention are also advantageous, in which helium is used as a cryogenic fluid, and the pump device is operated in such a way that in normal operation, the pressure in the cavity is between 100 mbar and 500 mbar, in particular between 150 mbar and 350 mbar, preferably between 200 mbar and 300 mbar.

An alternative method variant offers a high level of reliability for operating a cryostat arrangement having an activatable pump device, to which the cavity is connected via an activatable valve, and having a monitoring unit, which monitors the cooling function of the cold head and/or the pressure in the cavity, which is characterized in that the monitoring unit activates the pump device if the cooling function of the cold head drops and/or if the pressure in the cavity exceeds the predefined first threshold value P_(max) in such a way that the cavity is pumped out to a pressure below the threshold value P_(max).

Further advantages of the invention result from the description and the drawing. The above-mentioned features and the features listed hereafter can also each be used individually as such or in multiples in arbitrary combinations according to the invention. The embodiments shown and described are not to be understood as an exhaustive list, but rather have exemplary character for the description of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is illustrated in the drawing and will be explained in greater detail on the basis of exemplary embodiments. In the figures:

FIG. 1 shows a schematic cross section of a first embodiment of a cryostat arrangement according to the invention having a superconducting magnet coil directly in the vacuum as the object to be cooled;

FIG. 2 shows a schematic cross section of a second embodiment of the cryostat arrangement according to the invention having a cryogen container as the object to be cooled, wherein a superconducting magnet coil is contained in the cryogen container;

FIG. 3 shows a schematic cross section of a third embodiment of the cryostat arrangement according to the invention in the region of the neck tube having a pressure sensor connected to the cavity;

FIG. 4 shows a schematic detail of a fourth embodiment of the cryostat arrangement according to the invention having a cryopump; and

FIG. 5 shows a schematic time diagram of the heating of a cryostat arrangement according to the prior art in comparison to a cryostat arrangement according to the invention proceeding from an original operating temperature of 3 K after the failure of the refrigerating unit.

DETAILED DESCRIPTION

FIGS. 1 and 2 each show, in a schematic vertical sectional view, embodiments of the cryostat arrangement 1′; 1″; 1′″; 1″″ according to the invention having a vacuum container 2 and an object 4 to be cooled (in particular a superconductive magnet coil system 5 in an NMR, MRI, or FTMS apparatus), wherein the object 4 to be cooled is arranged inside the vacuum container 2, wherein the vacuum container 2 has a neck tube 8, which leads to the object 4 to be cooled, wherein a cooling arm 10 of a cold head 11 is arranged at least partially in the neck tube 8, wherein a closed cavity 9, which is sealed fluid-tight in relation to the object 4 to be cooled, is formed around the cooling arm 10, wherein the cavity 9 is at least partially filled with a cryogenic fluid in normal operation, and wherein a thermal coupling element 15 is provided, suitable for thermal coupling of the cryogenic fluid in the cavity 9 with the object 4 to be cooled.

The cryostat arrangement 1′; 1″; 1′″; 1″″ according to the invention is distinguished in that it comprises an activatable pump device 14, to which the cavity 9 is connected via an activatable valve 13 and with which the cavity 9 can be pumped out in the event of a drop of the cooling function of the cold head 11, and a monitoring unit 17 is provided, which monitors the cooling function of the cold head 11, and which is designed to independently activate the pump device 14 in such a way that the cavity 9 is pumped out if the cooling function of the cold head 11 drops.

FIG. 1 schematically shows a first embodiment of a cryostat arrangement 1′ according to the invention, comprising a vacuum container 2, in the interior of which a vacuum is configured. A thermal radiation shield 3 (shown by dashed lines), which encloses a superconducting magnet coil system 5 as the object 4 to be cooled here, is arranged here in the vacuum container 2. The magnet coil system 5 is arranged here directly in the vacuum of the vacuum container 2.

The cryostat arrangement 1′ is provided with a room temperature bore 6, through which a sample volume 7 in the center of the magnet coil system 5 is accessible. A strong, static, approximately homogeneous magnetic field B₀ prevails in the sample volume 7, which can be employed for NMR measurements on a sample in the sample volume 7 using NMR resonators (not shown in greater detail).

A neck tube 8 leads through the vacuum container 2 to the object 4 to be cooled. In the embodiment shown, the neck tube 8 simultaneously forms the border of a cavity 9, which directly encloses a cooling arm 10 of a cold head 11 of an active cooling system of the cryostat arrangement 1′.

The cavity 9 is connected via a pump line 12 and a valve 13, in particular a shut-off valve, to a pump device 14, with which the cavity 9 can be evacuated. A monitoring unit 17 is provided for activating the valve 13 and the pump device 14, which also receives (direct or implicit) items of temperature information from the cold head 11, and independently opens the valve 13 and activates the pump device 14 if a limiting temperature is exceeded.

During normal operation of the cryostat arrangement 1′, the cavity 9 is at least partially filled with a cryogenic fluid (not shown in greater detail in the drawing), which couples the cooling arm 10 to the object 4 to be cooled via a thermal coupling element 15. The thermal coupling element 15 is an upper side of the object 4 to be cooled here, which simultaneously forms a part of the border of the cavity 9. In the case of a disturbance of the active cooling of the cooling arm 10, the cavity 9 is pumped out using the pump device 14 and the cryogenic fluid in the cavity 9 is thus cooled.

FIG. 2 shows a second embodiment of a cryostat arrangement 1″ according to the invention, which substantially corresponds to the first embodiment of FIG. 1; therefore, only the essential differences will be explained.

In the second embodiment, the object 4 to be cooled is designed as a cryogen container 20, inside of which a superconducting magnet coil system 5 is arranged. Furthermore, a second cryogenic fluid, partially liquid and partially gaseous helium here (again not shown in greater detail in the drawing) is arranged in the cryogen container 20. The superconducting magnet coil system 5 is typically immersed at least partially in the liquid helium. The thermal coupling unit 15 is formed here by a part of the upper side of the cryogen container wall, which simultaneously delimits the cavity 9.

FIG. 3 shows a third embodiment of the cryostat arrangement 1′″ according to the invention in the region of the neck tube 8. In this—preferred—embodiment, a pressure sensor 30 is connected to the cavity 9. In refinements which are not shown separately in the drawing, the pressure sensor can in particular be arranged directly in the cavity 9. The output signal of the pressure sensor 30 is fed into the monitoring unit 17, which activates the pump device 14 to pump out the cavity 9 as soon the output signal of the pressure sensor 30 exceeds a predefined first threshold value P_(max), wherein in particular 100 mbar≤P_(max)≤500 mbar. After its activation because the first threshold value P_(max) is exceeded, the pump device 14 only pumps out the cavity 9 until the output signal of the pressure sensor 30 falls below a predefined second threshold value P_(min), wherein in particular 75 mbar≤P_(min)≤300 mbar.

In this embodiment of the cryostat arrangement 1′″ according to the invention, the pump device 14 comprises an electrically operated suction pump which is buffered by an autonomous power source, preferably with a battery 31.

Furthermore, this embodiment of the cryostat arrangement according to the invention comprises a device for refilling the cavity 9 with the cryogenic fluid. This device comprises a storage container 32 having the cryogenic fluid and a supply line 33, which connects the container 32 to the cavity. Using this device, the most autonomous operation possible can also be ensured after the failure of the cooling device, since the possibility exists of refilling the loss of cryogenic fluid. It is advantageous if the supply line 33 is equipped with a pressure reducer 34. The filling procedure can thus be carried out in a controlled manner without an overpressure arising.

FIG. 4 shows a fourth embodiment of the cryostat arrangement 1″″ according to the invention. The pump device comprises a cryopump 40 here—instead of an electrically operated pump—which is preferably integrated into the cryostat arrangement 1″″. The advantage of a cryopump 40 is that it can be embodied as completely passive and therefore very reliable.

A suitable adsorption material 41—for example, activated charcoal—is cooled in a cryopump. At a temperature of 4.2 K, even helium can thus be pumped. As shown in FIG. 4, the cryopump 40 can be integrated directly into the magnet cryostat. The pump is operated using a small quantity of liquid helium, which is indicated in the drawing below the adsorption material 41. The helium can evaporate into the atmosphere (1000 mbar), since it has a temperature of 4.2 K. The cryopump 40 cannot be thermally coupled well to the cold head, since heat is released during the pumping of helium using the cryopump 40, which would otherwise contribute to the heating of the magnet coil system 5.

One essential purpose of the present invention is to provide a cryomagnet which is operated, for example, at 3 K (corresponding to a pressure of approximately 240 mbar), in contrast to the conventional operation at 4.2 K (corresponding to the boiling point of helium at normal pressure). Due to this slight reduction of the temperature, the current carrying capacity rises in the superconductor, for example, NbTi, so that the design of a significantly more compact magnet is possible. According to simulations (not shown here), a magnet having a flux density of 9.4 T can thus be reduced from approximately 900 kg to 600 kg superconductor, because it can be charged with higher current densities.

It is disadvantageous in this case that the superconductor is more sensitive to an introduction of heat, and quench already occurs at lower temperature (approximately 4-5 K). A safety unit is thus required, with which to ensure that, in the event of failure of the cold head, the temperature of the cryostat can be kept as long as possible at 3 K.

It can be seen by way of example in the diagram shown in FIG. 5 how the temperature curve of helium develops upon failure of the cold head as a function of time. The MCV cryostat from the prior art cited at the outset (DE 10 2014 218 773 A1) can be used as an example (line having rhomboid symbols). After the failure, the temperature rises up to the boiling point at normal pressure, where the temperature has a plateau phase until the liquid component of the helium has passed into the gas phase. The temperature continually rises thereafter up to the quench.

Because of the present invention (line having square symbols), the temperature only rises until the limiting pressure (300 mbar/3.2 K) is reached. The pump subsequently begins to pump out gas in order to keep the bath at this temperature by vaporization. If all of the helium has vaporized, the temperature rises and a quench becomes unavoidable if the cold head fails for an even longer time.

In order to keep the temperature at 3 K, which is mentioned by way of example, in this manner, a pressure sensor 30 is necessary, as shown in FIG. 3. This pressure sensor 30 measures, in the case of rising temperature, the pressure increase in the cavity 9 linked thereto, where the cryogen (helium) is located, and relays the measured value to the monitoring unit 17, which activates the pump device 14 upon exceeding a threshold value P_(max), which then pumps out the cavity 9 to remove the cryogen from the gas phase until the desired (partial) vacuum is reached. In the case of accurate regulation of the partial vacuum, a regulating valve 13 (or a pump having variable speed) is advantageous.

During the charging or after failure of the cold head, the battery-buffered pump device 14 switches on upon exceeding the settable pressure threshold value P_(max) in the neck tube 8 and keeps the pressure stable through regulation, pressure sensor 30, and regulating valve 13 by pumping out helium. The pressure is somewhat above the pressure in normal operation. For example, 180 mbar in normal operation. Regulating pressure 300 mbar (corresponds to 3.2 K).

LIST OF REFERENCE NUMERALS

-   1′; 1″; 1′″; 1″″ cryostat arrangement -   2 vacuum container -   3 thermal radiation shield -   4 object to be cooled -   5 superconducting magnet coil system -   6 room temperature borehole -   7 sample volume -   8 neck tube -   9 cavity -   10 cooling arm -   11 cold head (=cold head) -   12 pump line -   13 valve -   14 pump device -   15 thermal coupling element -   17 monitoring unit -   20 cryogen container -   30 pressure sensor -   31 battery -   32 storage container -   33 supply line -   34 pressure reducer -   40 cryopump -   41 adsorption material

REFERENCE LIST

Publications considered for the judgment of patentability:

-   [1] DE 10 2014 218 773 A1 -   [2] U.S. Pat. No. 8,729,894 B2 -   [3] US 2007/089432 A -   [4] US 2010/298148 A -   [5] US 2007/022761A -   [6] DE 10 2004 012 416 B4 -   [7] US 2007/051115 A -   [8] U.S. Pat. No. 8,950,194 B2 

What is claimed is:
 1. A cryostat arrangement, comprising: a vacuum container containing an object to be cooled, wherein the vacuum container has a neck tube which leads to the object, a cooling arm of a cold head at least partially arranged in the neck tube, a closed cavity which is sealed off fluid-tight with respect to the object and is formed around the cooling arm and at least partially filled with a cryogenic fluid in normal operation, and a thermal coupling element configured to thermally couple the cryogenic fluid in the cavity with the object, a pump device, to which the cavity is connected via an activatable valve and configured to pump the cavity out in the event of a drop in cooling function of the cold head, and a monitoring unit configured to monitor the cooling function of the cold head, and to activate the pump device in response to a drop in the cooling function of the cold head such that the cavity is pumped out.
 2. The cryostat arrangement according to claim 1, wherein the object to be cooled comprises a superconducting magnetic coil system or a cryogen container.
 3. The cryostat arrangement as claimed in claim 1, further comprising a pressure sensor connected to the cavity and configured to output an output signal to the monitoring unit, wherein the monitoring unit is configured to activate the pump device to pump out the cavity as soon as the output signal of the pressure sensor exceeds a predefined first threshold value P_(max).
 4. The cryostat arrangement according to claim 3, wherein the pressure sensor is arranged in the cavity, and wherein 100 mbar≤P_(max)≤500 mbar.
 5. The cryostat arrangement as claimed in claim 3, wherein the pump device, following activation when exceeding the first threshold value P_(max), pumps out the cavity only until the output signal of the pressure sensor falls below a predefined second threshold value P_(min).
 6. The cryostat arrangement as claimed in claim 5, wherein 75 mbar≤P_(min)≤300 mbar.
 7. The cryostat arrangement as claimed in claim 1, wherein the activatable valve is configured as a regulating valve.
 8. The cryostat arrangement as claimed in claim 7, wherein the pump device is configured to operate at constant speed and/or constant pumping capacity.
 9. The cryostat arrangement as claimed in claim 1, wherein the pump device is configured to operate with variable speed and/or variable pumping capacity.
 10. The cryostat arrangement as claimed in claim 9, wherein the variable speed and/or the variable pumping capacity regulates pressure in the cavity.
 11. The cryostat arrangement as claimed in claim 9, wherein the activatable valve is configured as an ON/OFF valve.
 12. The cryostat arrangement as claimed in claim 1, wherein the pump device comprises an electrically operated suction pump buffered by an autonomous power source.
 13. The cryostat arrangement as claimed in claim 12, wherein the electrically operated suction pump is buffered with a battery.
 14. The cryostat arrangement as claimed in claim 1, wherein the pump device comprises a cryopump.
 15. The cryostat arrangement as claimed in claim 14, wherein the cryopump is integrated into the cryostat arrangement and comprises pumping cold surfaces that are thermally coupled to the object.
 16. The cryostat arrangement as claimed in claim 15, further comprising a connecting line that extends completely inside the vacuum container from the cavity to the pumping cold surfaces.
 17. The cryostat arrangement as claimed in claim 1, further comprising a supply line connected to the cavity and configured to refill the cavity with cryogenic fluid after the cooling function of the cavity is put back in the normal operation.
 18. A method for operating a cryostat arrangement comprising a vacuum container, an object to be cooled, and a thermal coupling element, wherein the object is arranged inside the vacuum container, wherein the vacuum container has a neck tube which leads to the object, wherein a cooling arm of a cold head is at least partially arranged in the neck tube, wherein a closed cavity, which is sealed off fluid-tight with respect to the object, is formed around the cooling arm, wherein the cavity is at least partially filled with a cryogenic fluid in normal operation, and wherein the thermal coupling element is configured to thermally couple the cryogenic fluid in the cavity with the object, comprising pumping the cavity out via a pump device such that the pressure in the cavity does not exceed a predefined first threshold value P_(max).
 19. The method as claimed in claim 18, wherein the cavity is pumped out via the pump device such that the pressure in the cavity does not fall below a predefined second threshold value P_(min)<P_(max).
 20. The method as claimed in claim 18, further comprising using helium as a cryogenic fluid, and operating the pump device such that in the normal operation, the pressure in the cavity is between 100 mbar and 500 mbar.
 21. The method as claimed in claim 20, wherein the pressure in the cavity is between 200 mbar and 300 mbar.
 22. The method as claimed in claim 18 for operating a cryostat arrangement, wherein the pump device is connected to the cavity via an activatable valve, and wherein a monitoring unit monitors the cooling function of the cold head and/or the pressure in the cavity, further comprising activating the pump device via the monitoring unit if the cooling function of the cold head drops and/or if the pressure in the cavity exceeds the predefined first threshold value P_(max) such that the cavity is pumped out to a pressure below the threshold value P_(max). 